The use of artificial discs in spinal surgery to restore spinal mobility in patients with degenerative disc disease has been practiced for several years as an alternative to spinal fusion, and commercial systems for such disc replacement are available. The artificial discs are generally made of a hard plastic material which can pivot between two generally metal end plates attached to the adjacent vertebrae. One of the problems associated with this procedure is the need to insert the disc assembly into the correct position between the two adjacent vertebrae, such that it provides the correct pivoting action to the adjacent vertebrae. Since an advantage of disc replacement over spinal fusion is the maintenance of mobility, correct pivoting action is of great importance.
Located between and behind each pair of adjacent vertebrae, are the facet joints, which are small joints which stabilize the segments of the spine relative to each other, but nonetheless preserve the flexibility needed to turn, bend, twist and generally ensure spinal mobility. Reference is made to FIGS. 1A and 1B which illustrates these facet joints 10, and the way in which, during spinal bending forwards (FIG. 1A) and backwards (FIG. 1B), the facet joints are stressed in one direction or the other. The arrows show the direction of motion of the top facet for these two bending situations. If the artificial disc is not accurately positioned, the neighboring vertebrae will not maintain their correct mutual positions, and undesired forces may be applied to the facet joints, even when the patient is static. This situation becomes even more aggravating in motion, when the facet joints have to operate dynamically, and as the spine bends forwards or backwards, excessive tension or thrust is put on the facet joints, generating pain for the subject. In some cases, the pain limits subject mobility to such an extent that spinal fusion occurs naturally, and the spine loses its flexibility in that region. Malpositioning of the disc may lead to sliding of the disk from it original location due to excessive lateral forces acting thereon. This often requires surgical revision and in some cases can cause severe nerve damage.
The optimum disc position is generally taken to be laterally on the midline of the vertebral body, and, at least as recommended for use with the Charité™ Artificial Disc supplied by DePuy International Ltd. of Leeds, U.K., 2 mm dorsal to the sagittal vertebral midline. According to current practice, the disc positioning is performed by the orthopedic surgeon by observing the operating region in real time on a series of fluoroscope images taken at different angles, generally laterally and A-P. The surgeon estimates visually from the fluoroscope images when the disc is in the optimum recommended position. However, this procedure may be difficult to perform accurately since fluoroscope images may be difficult to interpret clearly because of the large mass of intervening soft tissue, and the desired midline positions cannot be clearly delineated from these images. Furthermore, this procedure involves a high radiation exposure, both to the patient, the surgeon and the O.R staff.
Recently, a computer aided system for disc placement has been described by BrainLAB AG of Feldkirchen, Germany, in which a navigation system is adapted for use in defining the position of the artificial disc in relation to the neighboring vertebrae. The optical navigator uses sources or optical reflectors positioned on the vertebrae themselves, and on the disc insertion tool in order to relate the position of the disc to the vertebrae themselves. This may well be an improvement over visual position estimating methods, but it involves a complex navigation system, and maintenance of clear lines of sight during the entire operation. Since anterior access is used for these procedures, this may not be a simple requirement to fulfill.
The placement of artificial Spinal replacement discs is only one of a large number of surgical procedures in which objects have to be accurately positioned within the body. Amongst such objects are various other orthopedic implants, and surgical tools which have to be accurately inserted into their destined location to perform their intended task. Like disc replacement, such procedures are often performed under minimally invasive conditions, where site visibility or access is limited, thereby placing heavy reliance on the experience and skill of the surgeon to ensure proper placement of the object or tool.
There therefore exists a need for a surgical positioning system which overcomes at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.